Poop your suggestion seems to work, but only to a small extent and I don't know why. I made an image to further explain... I maxed out the threshold and it still doesn't affect all UV vertex groups, but only the three to the left in this case... weird!
Typically you can only weld UV verts that share the same GEO vert. Sometimes 3ds Max lets you do it, but this is not good practice. If the islands have the same face count and vertex order then you can try Copy / Paste. This is the best practice though, planning ahead: